(1) Field of the Invention
The present invention relates to a ball and socket type suspension insulator especially excellent in fatigue strength.
(2) Description of the Prior Art
Taking into account the interchangeability among the products of the manufacturers, the profile of the coupling portion between the ball pin and the socket of the ball and socket type suspension insulator is specified by International Electrotechnical Commission (IEC), etc. For instance, in the case of the suspension insulator of U210B (210 KN), the diameter (D) of the shank portion of the pin, the radius of curvature, (r), at the fillet portion between the pin shank portion and the ball portion, and the radius of curvature, (R), at the contact portion between the ball pin and the socket as shown in FIG. 2 are defined at 21 mm, 3.5 mm and 27 mm, respectively. When the fatigue limit of load amplitude is measured upon repeated application of various load amplitudes onto the suspension insulator having such a profile of the coupling portion while the average tensile load of 56 KN corresponding to 26.7% of the guaranteed electromechanical failing load (210 KN) is being applied thereonto, S-N curves as shown in FIG. 4 are obtained. The load amplitude at 2,000,000 repetitions corresponding to the fatigue limit of carbon steel which is ordinarily used in the ball pin of the suspension insulator is .+-.6.7% of the guaranteed electromechanical failing load, which is to be the fatigue limit of load amplitude of the conventional suspension insulator.
However, in the power transmission lines passing through the area where heavy snow and/or heavy ice attaches to the insulator or the wind pressure largely varies, the insulator has the drawback that since the load variation exceeding the above fatigue limit of load amplitude frequently occurs due to sleet jump when the attached snow and/or ice drops and galloping by wind, it can not maintain its mechanically supporting function over a long period of time, and therefore, a higher strength suspension insulator having one rank larger coupling size had to be used. In order to solve this drawback, various contrivances have been repeatedly done, for instance, changing of the material of the ball pin into a low alloy steel excellent in the mechanical strength, but the increase in the fatigue strength of the material affords substantially no improving effect on the fatigue strength of the ball pin. Further, trial has been done to mitigate the stress concentration through enlarging the radius of curvature, (r), at the fillet portion between the pin shank portion and the ball portion. However, a desired effect cannot be obtained unless the radius of curvature is set at a considerably large value, which increases the coupling size and unit length of the insulator. Thus, such countermeasure has been difficult to be practically applied.